By Brian Wise and Anthony S. Urciuoli
Summary: Due to advances in reverse osmosis membrane technology, hot water sanitization has gained increased usage. In certain cases, the relatively new technology serves as a practical alternative to traditional options. Some of the benefits are detailed here.
Reverse osmosis (RO) technology has been used for decades to generate pharmaceutical grade water. During this period, advancements in RO membrane technology and system design include polyamide thin film composite membranes, double-pass vs. single-pass RO systems, multi-stage vs. single-stage pumps, sanitary piping and instrumentation and, more recently, hot water sanitization membranes.
The performance requirements of RO systems used by biotech and pharmaceutical businesses are different from many market segments because, in addition to conductivity and total organic carbon (TOC), viable microbes and endotoxins are also measured in RO permeate water. The U.S. Pharmacopeia (USP) monographs stipulate water quality limits for “Purified Water” and “Water for Injection” (WFI). Table 1 provides an overview.
Although specific microbe limits aren’t defined, the USP recommends action levels for purified water and WFI. From a regulatory perspective, the end-user’s obligation is to ensure these limits are satisfied at the point of distribution. Advancements in membrane technology combined with pressure from regulatory agencies have increased RO system performance expectations, particularly regarding microbe and endotoxin measurements. Today, it’s common for end-user protocols to stipulate very low, viable microbe and endotoxin levels in RO permeate water.
Bacteria in membrane systems
A relatively large membrane surface area, ambient operating temperatures, and the absence of chlorine or other disinfectants combine to create a microbe breeding environment on the permeate side of RO membranes. To exhibit better system control over microbe levels, end-users have employed frequent, routine chemical sanitizing processes often based on the bacterial level measured in the permeate.
Chemicals used in RO membrane cleaning procedures include low and high pH-based cleaners for the primary purpose of removing hardness scale and organic deposits that may build up on the membrane surface as foulants. Some microbiological control is also achieved as a secondary benefit. Some common chemical polyamide membrane sanitizing agents include 0.5 percent formaldehyde, peracetic acid and hydrogen peroxide.
The USP and the recently published “ISPE Baseline Guide on Water and Steam Systems,” published by the International Society for Pharmaceutical Engineering (see www.ispe.com), allow end-users latitude in determining sanitization processes. Recently, chemical processes have yielded to hot water processes, due to the industry’s concern for introducing “added substances” into the water stream. This is because a chemical added to the water potentially could remain as a residual and find it’s way to the final product. With hot water, there’s nothing added—just heat—and this is viewed as being much more safe.
Shaking it ‘loose’
Bacteria counts taken from water streams are typically representative of the “loose” microbes in the stream and not necessarily of microbes that may exist in a biofilm within the water system. Biofilms are a complex organization of microbes formed by one of many common species, which adhere to a surface and cover themselves from the fluid stream in a somewhat protective poly-saccharide boundary layer.
Biofilms form naturally, but the growth may be mitigated with appropriate fluid velocity, smooth sanitary pipe designs and periodic and appropriate sanitization processes; however, biofilms have proven to be quite resilient to many chemical sanitization processes due to their protective boundary layer. Studies demonstrate biofilms may survive even after 60 minutes of exposure to chlorine-based sanitizing chemicals. Preventing the formation of biofilms, therefore, is a primary goal with respect to “regular and periodic” sanitization processes.
Although many chemical sanitization processes may be somewhat ineffective against biofilms based on specific chemicals, exposure time and frequency of application, the sub-boundary layer bacterial colony cannot escape exposure to sanitizing temperatures during a hot water sanitization process.
Hot water sanitization
It’s important to understand that “hot water RO” describes a system design for periodic sanitization with hot water, rather than an RO system operating at elevated temperatures. Effective sanitization with hot water is accomplished through an appropriate combination of exposure time and temperature. While not officially established, the industry has “standardized” 80°C (176°F) for 30 to 60 minutes as the optimum sanitization target point.
A primary use for hot water RO sanitization is to inactivate viable microbes. Endotoxin reduction isn’t achieved as a direct result of the hot water sanitization process. Based on the feed water source, system operating conditions and the end-user’s preventative maintenance practices, traditional chemical cleaning processes may still be required. Generally, hot water clean-ings are performed more frequently than chemical clean-ings, based on microbe counts and/or regular sanitization schedule.
Sanitization of an RO system using hot water commonly involves incorporating a heat exchanger(s) into the traditional clean in place (CIP) system to gradually, at a controlled rate, heat and cool water circulating through the membrane system. Membrane manufacturers commonly stipulate a controlled heating and cooling rate, 5°C per minute, to protect against irreversible damage to the membrane and ensure the system’s long-term performance (see Figure 1).
The recommended steps include:
- Ambient temperature and low pH cleaning for scale removal or other foulants as necessary,
- Heat to 80°C at a rate not to exceed 5°C per minute (approximately 30 minutes),
- Maintain feed pressure to RO system below 40 pounds per square inch gauge (psig),
- Maintain feed flow rate where pressure drop is less than 2 pounds per square inch differential (psid) per membrane,
- Circulate at 80°C (30 minutes), and
- Cool to 25°C at a rate not to exceed 5°C per minute (approximately 30 minutes)
When designing a hot water sanitization RO system, every water contact component must be considered to ensure temperature compatibility. Major components include membranes, membrane housings, pumps, pipes/fittings/gaskets, valves, instruments, and clean-in-place (CIP) tank.
Of these components, membranes are the most complex and costly. RO membranes used in hot water systems are similar to ambient temperature membranes in that they’re cast in polyamide material and constructed in a thin film composite configuration; however, to withstand the elevated operating temperatures, they’re manufactured with special adhesives, permeate tubes and connectors.
Hot water membranes
As with ambient temperature membranes, hot water membranes exhibit different operating characteristics at higher temperatures. For example, flux rates increase while rejection rates decrease (see Figures 1 & 2). When considering continuous operation at elevated temperatures, therefore, dissolved solids loading to post-RO treatment ion exchange equipment should be considered.
These operating parameters are essentially fully recoverable after the initial hot water sanitization at 55°C or higher. After the first sanitization, membrane flux at ambient temperature operation is permanently reduced by 10-30 percent nominally. This phenomenon is a factor of currently predominant membrane chemistry; however, subsequent hot water sanitizations have no further appreciable effect on ambient temperature flux rates. To ensure minimum flow requirements are satisfied in the long term, hot water systems are designed with significantly more membrane area as compared to ambient temperature systems with equivalent design flow rates.
System design options
There are several hot water sanitization membrane system designs that have been proven to operate successfully. Among the key and minimal design criteria for a dependable system are:
- Appropriate membranes,
- Appropriate materials of construction,
- Automated PLC/PID loop-controlled heating and cooling processes, and
- Automated valving to minimize operator interface.
Figures 3, 4 and 5 provide schematics for three basic system designs. In each design, a method to closely regulate feed pressure to the membranes during hot water sanitization is employed by either a pressure-regulating valve or a variable frequency drive for direct pump control. Figures 4 and 5 provide designs with independent heating and cooling heat exchangers. Figure 6 utilizes a common and dual-rated heat exchanger for both heating and cooling processes. In each case, the “hot” heat exchanger also serves to preheat RO feed water during normal operation.
It’s assumed Figures 3 and 4 utilize either sulfite chemical injection or ultraviolet (UV) technology for dechlorination. When using activated carbon (AC), as in Figure 5, both the RO and AC could be hot water sanitized during the same process. There are many plausible variations to hot water RO system design, so Figures 3, 4 and 5 shouldn’t be construed as exclusive options.
Hot water sanitization membranes and systems offer a very effective alternative to ambient temperature, chemically sanitized systems for maintaining very low viable microbe counts in RO permeate water. Today, membrane technology has advanced to the point where it can ensure reliable and predictable performance for use in high temperature systems and they can be automated to perform dependably and safely.
- United States Pharmacopeia 24 National Formulary 19, United States Pharmacopeial Convention Inc., Rockville, Md., 1999.
- Costerton, J.W., and P.S. Stewart, “Battling Biofilms,” Scientific American, July 2001.
- Meltzer, T., Pharmaceutical Water Systems, Tall Oaks Publishing Inc., Littleton, Colo., 1995.
- Snow, Michael J.H., et al., “New Techniques for Extreme Conditions: High Temperature Reverse Osmosis and Nanofil-tration,” Desalination 105, 1996.
About the author
Brian Wise is the application engineering manager for engineered equipment at Osmonics Inc., of Minnetonka, Minn. He can be reached at (952) 988-2277 or email: firstname.lastname@example.org. Anthony S. Urciuoli is sales engineer for the Pharmaceutical Systems Group at Osmonics Inc. He can be reached at email@example.com